1. Field of the Invention
The present invention relates to piezoelectric ceramic materials, sintered piezoelectric ceramic compacts using the same and piezoelectric ceramic devices formed of the sintered piezoelectric ceramic compacts, and more particularly, relates to a sintered piezoelectric ceramic compact advantageously used for forming a piezoelectric ceramic device which is required to have a relatively low electromechanical coupling coefficient and superior temperature stability of the resonant frequency, and to a piezoelectric ceramic material for forming the sintered piezoelectric ceramic compact.
2. Description of the Related Art
Heretofore, sintered piezoelectric ceramic compacts formed by firing a piezoelectric ceramic material composed of lead titanate zirconate, i.e., PbZrO3-PbTiO3, and a lead-based complex perovskite compound, i.e., Pb(MaxMdy)O3, dissolved therein (in which Ma is at least one divalent element or trivalent element, and Md is at least one pentavalent element or hexavalent element) have been used for forming filters or oscillators using bulk waves or surface waves.
Since a sintered piezoelectric ceramic compact formed by firing a piezoelectric ceramic material containing Mn as Ma, which is an acceptor component, has a relatively low loss among sintered lead titanate zirconate-based piezoelectric compacts, the sintered piezoelectric ceramic compact described above has been widely used in various applications.
In addition, trials for variously modifying the compositions of piezoelectric ceramic materials have been conducted in order to improve the resonant properties of these sintered piezoelectric ceramic compacts.
For example, it has been disclosed in Japanese Unexamined Patent Application Publication No. 5-327397 that when a piezoelectric ceramic material represented by the formula (Pb1xe2x88x92xMex){(Mn⅓Nb⅔)aTibZrc}O3 is used, in which Me is at least one selected from the group consisting of Ca, Ba and Sr, and x, a, b and c satisfy 0.005xe2x89xa6xxe2x89xa60.10, 0.01xe2x89xa6axe2x89xa60.14, 0.40xe2x89xa6bxe2x89xa60.60, 0.26xe2x89xa6cxe2x89xa60.59, and a+b +c=1.00, a surface wave device can be produced having superior resonant properties, temperature dependence thereof, and heat stability.
In addition, in Japanese Unexamined Patent Application Publication No. 5-24916, a material is disclosed which contains at least one of SiO2 at a content of from 0.005 to 0.040 wt % and Al2O3 at a content of from 0.005 to 0.040 wt % as a {PbSr}{(TiZr)(MnNb)}O3-based piezoelectric ceramic material which reduces variation in electrical properties.
Sintered piezoelectric compacts formed of these piezoelectric materials have relatively superior properties, such as resonant properties and temperature stability thereof, and hence, they can be advantageously used for various industrial applications.
However, when a conventional sintered piezoelectric ceramic compact having relatively superior temperature properties, as described above, is used for forming a narrow bandwidth filter, which is required to have better temperature properties than a wide bandwidth filter, a problem may occur in some cases in that the central transmission wavelength in the filter varies considerably.
In addition, since the conventional sintered piezoelectric ceramic compact described above has a relatively high electromechanical coupling coefficient in a state in which the degree of polarization is saturated, and hence, in the state described above, there is a problem in that a narrow bandwidth filter or a highly precise oscillator, both of which are required to have a small difference between the resonant frequency and the antiresonant frequency, cannot be provided.
Furthermore, in order to decrease the electromechanical coupling coefficient when the degree of polarization is unsaturated, degradation of the resonant resistance or an increase in variation of the degree of polarization occur, and as a result, insertion losses of filters may be increased, oscillations of oscillators may become unstable or the production yield may be decreased.
In order to solve the problems described above, it has been disclosed that the variation of degree of polarization can be decreased by appropriately designing a polarization method for polarizing piezoelectric compacts in U.S. Pat. Nos. 2,783,022, 2,890,863, and Japanese Examined Patent Application Publication No. 7-105684.
According to the methods disclosed in these publications, the degree of polarization can be effectively decreased; however, degradation of the resonant resistance cannot be avoided when the degree of polarization is decreased. Consequently, when the methods mentioned above are applied to a piezoelectric compact for forming a filter or an oscillator, the insertion loss of the filter may be increased, or the oscillation of the oscillator may become unstable in some cases. In addition, a polarization step must be performed at least two times in manufacturing piezoelectric products, and as a result, the time required for manufacturing the piezoelectric products is disadvantageously the prolonged.
Accordingly, an object of the present invention is to solve the problems described above, and more particularly, is to provide a piezoelectric ceramic material having preferable properties for designing narrow bandwidth filters or highly precise oscillators, a sintered piezoelectric ceramic compact formed by firing the piezoelectric ceramic material and a piezoelectric ceramic device comprising the sintered piezoelectric ceramic compact.
A piezoelectric ceramic material according to the present invention comprises a solid solution having a primary component composed of PbTiO3, PbZrO3 and Pb(MaxMdy)O3, in which Ma is at least one selected from the group consisting of bivalent elements and trivalent elements, and Md is at least one selected from the group consisting of pentavalent elements and hexavalent elements, wherein the solid solution has a tetragonal crystal system, and a ratio X/Y, which is the ratio of the Ma content X to the Md content Y, is larger than the stoichiometric ratio.
When a piezoelectric ceramic material having a predetermined composition and a predetermined crystal system is specifically selected, superior temperature stability of the resonant frequency of a sintered piezoelectric ceramic compact which is formed by firing the piezoelectric ceramic material described above can be obtained. Compared to the case in which the X/Y coincides with the stoichiometric ratio, the temperature coefficient of the resonant frequency can be decreased. When the crystal system is other than a tetragonal crystal system, even when the X/Y is increased to more than the stoichiometric ratio, the effect of improving the temperature properties cannot be obtained.
In the piezoelectric ceramic material of the present invention, it is preferable that Ma be Mn, and Md be at least one selected from the group consisting of Nb, Sb, Ta and W. When the contents of Mn, Nb, Sb, Ta and W are represented by A, B, C, D and E, respectively, the ratio of acceptor to donor, i.e., A/(B+C+D+2E), preferably satisfies 0.525xe2x89xa6A/(B+C+D+2E)xe2x89xa61 on a molar basis.
When 0.525xe2x89xa6A/(B+C+D+2E)xe2x89xa61 is satisfied, the effect of improving the temperature stability of the resonant frequency of the sintered piezoelectric ceramic compact obtained by firing the piezoelectric ceramic material can be more significant, and a polarization treatment can be more easily performed. When A/(B+C+D+2E) is less than 0.525, the effect of improving temperature properties cannot be obtained, and in contrast, when A/(B+C+D+2E) is more than 1, the electric insulating properties of the sintered piezoelectric ceramic compact is degraded, and a polarization treatment may be difficult to perform in some cases.
In the piezoelectric ceramic material of the present invention, it is preferable that from more than zero to about 20 mole percent of the element Pb contained in the primary component described above be replaced by at least one selected from the group consisting of Ca, Ba, Sr and La.
When replacement is performed as described above, the effects of decreasing variations in electromechanical coupling coefficient, resonant frequency, and degree of polarization of the sintered piezoelectric ceramic compact obtained by firing the piezoelectric ceramic material can be more significant compared to the case in which replacement is not performed. In addition, when about 1 to 4 mole percent of the Pb is replaced, the effect of decreasing variations can be further improved. In this connection, when more than about 20 mole percent of the element Pb is replaced, the Curie temperature of the sintered piezoelectric ceramic compact is decreased, and as a result of the temperature used in machining or processing for forming a piezoelectric ceramic device, the piezoelectric properties of the sintered piezoelectric ceramic compact may disappear in some cases.
In the piezoelectric ceramic material of the present invention, about 0.003 to 0.1 parts by weight of SiO2 and about 0.003 to 0.1 parts by weight of Al2O3 are preferably contained with respect to 100 parts by weight of the primary component.
The contents of SiO2 and Al2O3 described above serve to improve the strength of the sintered piezoelectric ceramic compact obtained by firing the piezoelectric ceramic material. When the contents of SiO2 and Al2O3 are increased to more than about 0.1 parts by weight, respectively, undesired resonant resistance of the sintered piezoelectric ceramic compact may be increased in some cases.
The present invention can also be applied to a sintered piezoelectric ceramic compact formed by firing the piezoelectric ceramic material described above. In particular, this sintered piezoelectric ceramic compact is obtained by firing the piezoelectric ceramic material of the present invention in an oxygen atmosphere at an oxygen concentration of about 80 percent by volume or more.
The sintered piezoelectric ceramic compact is preferably used in an unsaturated polarized state having an electromechanical coupling coefficient corresponding to about 80% or less of that of a sintered piezoelectric ceramic compact in a saturated polarized state.
The present invention can also be applied to a piezoelectric ceramic device comprising the sintered piezoelectric ceramic compact described above and electrodes provided on surfaces of the sintered piezoelectric ceramic compact. As such piezoelectric ceramic devices, for example, a filter, a trap device, or an oscillator may be mentioned.